The present invention relates to power plats and pertains particularly to pressurized circulating fluidized bed (PCFB) boiler power plants for operating at supercritical steam pressures.
There exists in the power generating industry an ever increasing need for more efficient power plants for converting fossil fuels to electrical power. This need continues to increase as the cost and scarcity of clean burning conventional fuels becomes even greater. This requirement for more efficient plants has led to the development of supercritical boiler designs for some large conventional power plants.
Supercritical operation is at pressure above 3208 psi so that steam does not separate from the liquid, i.e. a single phase fluid. Supercritical boiler designs have been used in fossil fuel fired conventional power plants. These large conventional power plants typically have furnace pressure very close to the atmosphere pressure.
The major concern in designing supercritical boilers is to establish and maintain sufficient water mass flow through the combustor wall tubes under all operating conditions. This is complicated by the presence of the flame in the conventional boiler combustors. The presence of flame in the combustor produces a high heat flux to the water walls and hence a higher mass flow is required through the tubes to keep the tube wall temperatures low.
The need for higher efficiency plants is even greater for convening lower grades of sulfur containing fuels, such as coal, that exist in abundance in many regions of the world. These lower grades of fuel create atmospheric pollution when burned in conventional combustors. Many of these fuels contain impurities, such as sulfur which reacts in the combustion process forming compounds such as SO.sub.2 that is particularly noxious and pollution. Systems, including scrubbers, have been developed for removing these pollutants from exhaust gasses of power plants. However, these systems are very expensive and frequently not cost effective for most power plants.
Circulating fluidized bed combustors have been developed in recent years for burning sulfur containing fuels to generate steam for powering steam turbines. The circulating fluidized bed combustor has been further improved by pressurization of the combustor. The pressurized circulating fluidized bed combustor operates at pressures substantially above atmospheric pressure with a mixture of granular limestone or other sorbent materials supported on a non-sifting grid. An upward flow of pressurized air passes through the grid lifting and fluidizing the material. This results in a turbulent mixture of the bed particles having the free flowing properties of a liquid and providing an environment for stable combustion. Fuels introduced into the bed will burn effectively, and sulfur dioxide released by the burning is chemically captured by the calcined limestone. The mixture of solids which includes ash and calcined limestone is recirculated through the combustor until the particle size is reduced sufficiently for elution through the cyclones.
As sulfur containing fuel is burned, the sulfur combusts with oxygen to form sulfur dioxide. The limestone is calcined by the combustion temperatures, and the sulfur dioxide then reacts with the calcium oxide and oxygen to form calcium sulfate. Sulfur removal depends on contact between the sulfur dioxide molecules and the calcium oxide particles.
Applicant has discovered and developed an arrangement whereby a pressurized fluidized bed combustor (PCFB) for burning sulfur containing fuels is constructed to operate at supercritical steam pressures. The pressurized circulating fluidized bed combustion chamber operates at elevated pressures considerably above atmospheric. The PCFB boiler has some advantages that lend itself to avoid the complications of the conventional boiler. These include smaller cross section combustors for the same heat duty. The number of wall tubes required is less, so the required mass flow through the tubes could be easily maintained.